What if both players only bring Isles of Fire MI’s? (no islands can be HI’s) Would you have to just ignore the ability text for all of them?
Isle of Fire: This island can be used only as a wild island. At the beginning of his or her turn, the active player rolls a d6. On a result of 6, randomly eliminate one treasure coin from this island.
The best option is to ignore the effect for the ones chosen as home islands, or swap the ones chosen as home islands with non-fire islands.
You could also require each player to provide at least one non-fire island, although that could force some extremely unbalanced home island locations.